Urokinase, the well known plasminogen activator produced by renal cells and urinary tract is excreted in the urine and has been the subject of much interest for possible use as a medicament against sudden fibrinolytic disturbances such as thromboembolism.
One major set of difficulties hindering wide spread employment of urokinase are the problems encountered in the recovery thereof. Human urine, the first recognized source of urokinase contains a low proportion of the enzyme, and therefore human urine for source material purposes offers extreme logistic problems as well as concern that some of the urine donors might also be excreting viruses or other detrimental substances in the urine.
Human kidney cells cultured in vitro, are another recognized source of plasminogen activator. Here too, the yield is relatively low, although yield improvement techniques (c.f. U.S. Pat. Nos. 3,930,945 and 3,904,480 for example) have resulted in limited availability of the kidney cell product.
Since activators derived from transformed, heteroploid or continuous cell lines are not acceptable for production of pharmaceutical products due to possible presence of oncogenic agents and nucleic acid fragments in the products, the normal embryonic kidney cells constitute heretofore the best source material for plasminogen activator. However, normal human embryonic kidney cell cultures have a limited in-vitro life span (20 population doublings) and a supply of fresh embryonic kidney cells is required at frequent intervals. Human embryo kidney is of limited availability, closely controlled by government regulations in most countries. Universally, obtaining viable (embryo) kidney cells involves emotive questions. Whether normal embryonic kidney cells can ever be obtained in the numbers and regularity required to sustain large scale production of plasminogen activator is open to serious question. Nonetheless, heretofore kidney cells cultured in vitro represented the best hopes of the art as source of urokinase in usable quantities.
In the early 1960's Dr. L. Hayflick of the Wistar Institute developed strains of normal human lung cells. These cells, of fibroblastic morphology and exhibiting normal diploid karyotype, enter senescence after 40-60 population doublings. By this method, a pool of cells derived from the lung tissue of a single normal foetus can be screened for the presence of adventitious agents, normal karyotype etc. If satisfactory, this pool can be passaged through 30 further population doublings for the production of biological products such as vaccines. This system gives very large culture areas and has proved extremely satisfactory in the production of polio, measles, mumps, rubella and other vaccines. At least two such cell types have received official approval (by WHO) for the production of biological products, (i.e. MRC-5 and WI-38) and others are under development (eg IMR-90).